With enough Snow mana, you can also pump it up multiple times for extra damage
And yet, with all this reasoning that it isn't a good idea, I'm not sure if it's something worth actually caring about
If youre doing this, you may also want a way to untap Master Transmuter so that you can return the Silent Arbiter to your hand before your next Declare Attackers phase, so you can use the same trick again next turn (but so your opponents can only have one attacker).
The flexibility of being able to choose a value for X is great because we might have an early-game case of wanting (or needing) to cast it for X of one or two, or we can wait until late-game and sweep a much bigger battlefield
Menace and ward are going to help you attack profitably and avoid removal, at least somewhat
Its no easy feat milling out 100 cards, let alone when youre trying to do it to multiple opponents